Transition Metals, Alloys And Corrosion - Topic 5 Flashcards
What are most metals?
- most metals are transition metals
Physical properties of transition metals:
- high mp and bp
- formation of coloured compounds
- catalytic activity of the metals and their compounds as exemplified by iron
- ductile
- malleable
- strong, durable
- shiny
- dense
- conducts electricity and heat
- solid at RTP
- can form more than 1 type of ion
What does a catalyst do?
- transition metals are good catalysts
- a catalyst is a substance that increases the rate of reaction by providing the reaction with an alternate reaction pathway that has a lower activation energy without being chemically changed or used up
Colour of copper compound:
blue
Colour of iron(II) compound:
pale green
Colour of iron(III) compound:
orange/brown
Colour of nickel compound:
green
Colour of chromium compound:
dark blue/green
Colour of cobalt compound:
pink - when it absorbs water
How could you prove that all the elements in the transition block are metals and not non-metals?
- test conductivity of electricity - lump of elements can complete a circuit and light the bulb
- metals will conduct electricity and non-metals won’t conduct electricity
Why is FeCl2 called iron(II) chloride and FeCl3 is called iron(III) chloride?
Roman numerals give charge of transition metals
Transition metal:
a metal found between Group 2 and 3 of the periodic table
What does the oxidation of metals result in?
oxidation of metals results in corrosion
Corrosion:
- destruction of materials by chemical reactions with substances in the environment
- e.g. iron rusts when exposed to water and oxygen
Example of corrosion:
- rusting - both air and water are necessary for iron to rust
- i.e. oxidation - gain of oxygen results in corrosion
How rusting of iron can be prevented:
- exclusion of oxygen
- exclusion of water
- sacrificial protection
Methods of preventing the rusting of iron:
- painting the iron surface
- galvanising
- sacrificial protection
- stainless steel
How does painting the iron surface prevent iron from rusting?
- applying oil and grease, covering in plastic
- barrier method - stops O₂ and H₂O reaching iron and reacting with it
Potential problem of painting the iron surface to prevent the iron from rusting:
- cover the iron
- however, when the barrier is broken the iron will rust
Examples of painting the iron surface to prevent the iron from rusting:
- bicycle chains (oil)
- bicycle brake cables (plastic)
- bridges (paint)
How does galvanising the iron prevent iron from rusting?
- covering steel with zinc - still works even if zinc is scratched away at some places
- zinc replaced once is is worn down
Potential problems of using galvanising to prevent the iron from rusting:
- need to keep checking if there is enough zinc left on the iron
Examples of using galvanising to prevent the iron from rusting:
dust bin and some nails
How does sacrificial protection work to protect iron from rusting?
- where the metal you want to be protected from rusting is galvanised with a more reactive metal, which will rust first and prevent water and oxygen reaching the layer underneath
- e.g. zinc used to iron
Potential problems with using sacrificial protection to prevent iron from rusting:
- more reactive metal can be chipped and iron will still not rust
- need to keep checking if there is enough zinc left on the iron
Examples of using sacrificial protection to prevent iron from rusting:
- ships
- underground pipes
How is stainless steel used to prevent iron from rusting?
- the chromium in stainless steel reacts with oxygen in the air to produce a protective chromium oxide layer across the surface
- acts as a protective barrier
Potential problems with using stainless steel to prevent iron from rusting:
more expensive
Examples of using stainless steel to prevent iron from rusting:
cutlery
What can electroplating be used to do?
can be used to improve the appearance and/or the resistance to corrosion of metal objects
How can a metal object be covered with a coating of another metal?
- if an iron object is completely covered with a coating of another metal the the iron will not rust - can be done by plating the object
- often the metal coating can improve the appearance of the iron e.g. chromium plating
Electroplating:
the process of coating a metal with a thin layer of another metal by electrolysis to improve the metal’s corrosion resistance or to improve the metal’s appearance
Example of electroplating:
- chromium plating covers the iron object with a thin layer of chromium
- this stops the iron from rusting and gives the object a very shiny appearance
How does electroplating work?
- Electroplating acts as a barrier in order to exclude oxygen and water
- Also improves appearance as you can electroplate a metal with an unreactive metal such as gold that is more attractive and will not corrode
- The object to be plated is made the cathode (i.e. is given a negative charge) and dipped into a solution containing ions of the desired plating metal
How does copper plating work?
- Connect the object to be plated to the negative terminal of a power supply using a wire and a crocodile clip
- Use a wire to connect a strip of copper to the positive terminal
- Place both objects into a solution of copper sulphate and turn the power supply on
- A lower voltage and rotation of the iron object usually gives the best results
Diagram of copper plating:
What is the solution used in copper plating?
copper sulphate
What object is the cathode in copper plating?
the object being plated
Write a half-equation for the reaction at the cathode in copper plating. What type of reaction is this?
- Cu²⁺ + 2e⁻ ——> Cu
- reduction
What object is the anode in copper plating?
impure copper
Write a half-equation for the reaction at the anode. What type of reaction is this?
- Cu ——> Cu²⁺ + 2e⁻
- oxidation
Alloy:
- a mixture of metals containing 2 or more metals together
- process is carried out to give the material greater strength, greater resistant to corrosion or a better appearance
What metals are mixed with other metals and why?
pure copper, gold, iron and aluminium are too soft for many uses and so are mixed with small amounts of similar metals to make harder alloys for everyday use
What are most metals in every day use?
most metals in every day use are alloys
Why are allows harder than pure metals?
- this works because in a pure metal, all the + metal ions are the same size and in a regular arrangement, allowing the layers to slide over each other relatively easily when a force is applied → makes the metal soft and malleable
- in an alloy, you have + ions of different metals, which have different sized ions
- this disrupts the regular structure and prevents the ions being able to slide as easily, leaving a much harder, stronger metal
What is steel an alloy of?
- iron and small amount of carbon
- other metals can be added to give it different properties
- e.g. copper and nickel can be added to make stainless steel - doesn’t rust
Why is iron alloyed with other metals to produce alloy steels
- Some steels contain other metals
- Alloys can be designed to specific uses
Why is iron alloyed to form mild steel?
- made up of 99.5% iron and 1% carbon
- mild steel is easily worked, malleable and strong as it has lost most brittleness
- it is used in car bodies, machinery and sheeting
Why is iron alloyed to form hard steel?
- made up of 99% iron and 1% carbon
- hard steel is tough and brittle
- it is used in power shovels ~~and cutting tools~~
Why is iron alloyed to form stainless steel?
- made up of iron, chromium and nickel
- stainless steel is tough and resistant to corrosion
- it is used in chemical plants and cutlery
Why is iron alloyed to form tungsten steel?
- made up of iron and tungsten
- tungsten steel is tough and hard - even at high temperatures
- it is used in micro-drills
Properties of aluminium:
- low-density
- malleable
- low reactivity
- low toxicity
Uses of aluminium:
- aircraft parts
- cooking foil - malleable, low toxicity, low reactivity
Properties of copper:
good conductor
Uses of copper:
electrical cables
Properties of gold:
- good resistance to corrosion
- good electrical conductor
- attractive appearance
Uses of gold:
- used in jewellery
- electrical contacts - doesn’t corrode + food electrical conductor
What is brass an alloy of?
an alloy of copper and zinc
Properties of brass:
- hard
- copper made harder by addition of zinc
- in brass the layers of the copper atoms cannot slide over each other as easily as larger zinc atoms get in the way
- resistant to corrosion
- looks good
- strong and long lasting
Uses of brass:
- musical instruments - looks good and doesn’t corrode
- door handles - doesn’t corrode
- pins for electrical plugs - strong and hard wearing
What is magnalium an alloy of?
magnesium and aluminium
Properties of magnalium:
- strong
- resistant to corrosion
- low density
- flammable
Uses of magnalium:
- cars
- aircrafts
What is electrum an alloy of?
silver and gold
Uses of electrum:
- coins
- jewellery
What is bronze an alloy of?
copper and tin
Properties of bronze:
- harder than copper
- less malleable
